It is more particularly related to an implementation of cooperative communications making use of dynamic relays in a mobile network, notably a cellular network in which base stations have a radio coverage over respective cells of the network. With reference to FIG. 1 illustrating the situation of a cellular network, a cooperative communication denotes a transmission technique where:                in downlink mode DL, terminals TER receive useful signal from a base station BS via other terminals REL1 (one or more) which relay the useful signal transmitted by the base station BS, and        in uplink mode UL the base station BS receives useful signal coming from a transmitter terminal TER via other terminals REL2 (one or more) which relay the transmitted signal to the base station.        
For clarity of presentation in FIG. 1, only the links via relays REL1, REL2 are shown. In practice, however, the same signals can be transmitted via various paths:                via relays (as shown in FIG. 1), and        directly between the base station BS and the terminal TER.        
With for example two signals carrying the same information and coming from different nodes of the network (coming for example directly from the base station, on the one hand, and from one or more relays, on the other), this diversity of communication “paths” taken allows the signal-to-noise ratio on reception of the signals to be improved.
Cooperative communications in a cellular network allow the terminals (or the base stations, respectively) to receive useful signal from the base station (or from the terminals, respectively) and also from other nodes of the network (typically relay nodes) which relay the useful signal.
A transmission that makes use of the relay nodes must use a relay protocol. The most well known relay protocols are:                the protocol “Amplify-and-Forward” (AT), according to which the relay amplifies the signal that it receives and retransmits the signal thus amplified, and        the protocol “Decode-and-Forward” (DT), according to which the relay decodes the signal received before retransmitting the signal, which allows the noise initially present in the received signal to be eliminated.        
The relay nodes in a cellular network may be static or dynamic. Static relay nodes belong to the infrastructure of the network and they are deployed in particular chosen points of the cell. Dynamic relays are generally mobile nodes which can for example be terminals that are situated within a given cell and which behave as relay nodes by assisting the transmission toward one destination (or more), where this destination can be a terminal (“downlink” case) or a base station (“uplink” case).
Cooperative communications (use of the relay nodes) within a cellular network can increase the coverage of the cell, improve the reception of the signals (by offering a gain in diversity for example) and increase the spectral efficiency of the network (which is defined in bits per second, per Hertz and per cell). The easiest way of making use of relays is simply to deploy a few static relays at particular points of the cell. This practice makes the selection of the relays, for a particular transmission, reasonably easy because the potential relays are limited.
The selection of the relays participating in a particular transmission is obligatory in a system where many relays (either static or dynamic) may exist. The most relevant relays and those truly providing a gain in performance when they assist a transmission must therefore be chosen. Furthermore, it could be that, in certain situations, it is impossible to find relevant relays, such that the direct transmission between the source and the destination must be imposed. Moreover, if static relays are deployed, the points of deployment of the relays may be chosen so as to guarantee a sound link between the relays and the base stations, which can enhance the efficiency of the relay operation.
However, the deployment of the static relays increases the cost of the systems because it requires other infrastructure elements (relay nodes) to be added. In contrast, the implementation of dynamic relays does not require an additional infrastructure cost. The user terminals available in a cell can relay signals in order to assist the transmission to other terminals. Dynamic relays may be applied both in the downlink direction and in the uplink direction. Nevertheless, the use of dynamic relays poses problems of complexity and of loading on the return channel. The return channel denotes a transmission from the terminal to the base station intended to supply control information on the terminal to the base station, such as for example the quality of its channel.
The information transmitted over the return channel represents a loss of spectral efficiency for the system, since it occupies radio resources which cannot be used for the transmission of useful signals. Indeed, in order to select the relay (or relays) to ensure the transmission to a destination, the base station (an entity that usually takes such decisions in a cellular system) must acquire information on the state of the transmission channels (or “CSI” for “Channel State Information”) between the source and the potential relays, and also between the potential relays and the destination. The CSI information may for example consist of channel transmission coefficients or of other, more concise, information on the quality of the channel, such as for example the gain of the channel or a signal-to-interference and signal-to-noise ratio, or other types.
Hereinafter, “gain of the channel” will be understood to mean the ratio between the power received at the receiver antennas and the power emitted by the transmission antennas. This gain depends:                on the propagation losses (or “pathloss”), which increase with the distance between a transmitter node and a receiver node,        on the “shadowing” effect,        and also on the “fast fading” of the channel.        
Hereinafter, “attenuation” is also understood to mean the cumulative effect of the pathloss and of the “shadowing” effect; in practice, the attenuation can be estimated for example by the inverse of the gain of the channel (and then depends on the fast fading). By estimating an average of the inverse of the channel gain over several successive values, the dependency of the estimated attenuation on the fast fading can be reduced.
The transmission of the information on the state of the transmission channels CSI increases the load on the return channel, together with the complexity of the process of selection of the relays. This is because the load on the return channel and the complexity of the selection process are proportional to the number of candidate terminals for the relay operation. No technique is currently known for reducing the number of potential relays capable of assisting the transmission to a terminal or a base station. Thus, the number of candidate relay nodes for a particular transmission is not currently reduced and the complexity of the selection, together with the load on the return channel, makes the relay operation difficult in a cellular network.
In the framework of ad-hoc networks employing multihop transmission, a technique has been developed for the selection of relay nodes in:
“Geographic Random Forwarding (GeRaF) for Ad Hoc and Sensor Networks: Multihop Performance”, M. Zorzi and R. R. Rao, IEEE Transactions on Mobile Computing, Vol. 2, No. 4, Pages 337-348 (October-December 2003).
According to this technique, the node transmitting data (“source node”) knows its geographical coordinates and the coordinates of the node that wants to receive the message from the source (“destination node”). The source transmits a packet which contains its coordinates and the coordinates of the destination node. All the nodes that receive the packet estimate their distance to the source and to the destination, basing this on the received coordinates. The nodes that are closer to the destination than the source become candidates for the relay operation. In order to define the relay, a priority level is assigned to each candidate node according to their proximity to the destination, and the node having the highest priority (the closest to the destination) is finally selected. This technique is targeted for ad-hoc networks and does not take into account the shadowing effect, which is however essential for cellular networks, nor the fast fading of the channel. Moreover, this technique requires the nodes to use a receiver of the GPS (for “Global Positioning System”) type in order to estimate their geographical coordinates, a fact which increases the cost of the system.
One technique for the selection of the best relay between a source and a destination based on the quality of the instantaneous source-relay and relay-destination channels has been developed in:
“A Simple Cooperative Diversity Method Based on Network Path Selection”, A. Bletsas, A. Khisti, D. P. Reed and A. Lippman, IEEE Journal on Selected Areas in Communications, Vol. 24, No. 3, Pages 659-672 (March 2006).
In this technique, a source sends an RTS packet (called “Ready-To-Send” in this document) and all the nodes in the neighborhood estimate the gain of the channel between the source and themselves. This packet is detected by the destination node that sends a CTS (“Clear-To-Send”) packet, which allows the nodes in the neighborhood to estimate the gain of the channel between the destination and themselves. After the reception of the CTS packet, the potential relays trigger a timer that expires depending on the current quality of the two channels (source-relay and relay-destination). Thus, the relay that has the timer which expires the fastest is the best relay and it sends a flag packet. This flag packet is then detected by other candidate relays which stop their timers.
This particular technique does not take into account the structure of the cellular networks that may be exploited to simplify the relay selection method.
The techniques developed by Zorzi et al (2003) and Bletsas et al (2006), which are really designed for ad-hoc networks, are therefore based on the principle that the transmission between the source and the destination is always assisted by a relay node (where potential relays exist). However, the use of a relay does not always increase the capacity of the transmission. Accordingly, in a real system, a relay must be used only if it truly enhances the performance of the transmission.
The performance of the transmission can be estimated for example on the basis of a capacity of the transmission making use of a particular relay, where the capacity of the transmission can then be a performance criterion. “Capacity of the transmission” is taken to mean, depending on the chosen metric, the data rate offered for the transmission, or its quality of service (or “QoS”), or again its signal-to-noise ratio, or any other criteria of this type. It should then be pointed out that, for example for the criteria on data rate offered, a relay results in a loss of resource, in terms of resource allocation time interval (or “TTI” for “transmission time interval”), which decreases the total data rate of the system.
For example, for a transmission in “half-duplex” mode (since the relay does not transmit simultaneously in uplink and in downlink mode), the use of a relay requires two transmission time intervals (“TTI”):                one for transmitting from the source to the relay, and        the other for transmitting from the relay to the destination.        
The period TTI is conventionally the elementary interval of time during which radio resources can be allocated to a terminal. Therefore, the capacity of a transmission attained with the aid of a relay node is divided by a factor of two in the “half-duplex” mode of operation. Thus, the performance of the relay operation in “half-duplex” mode is limited by this factor. As a result, in all the modes of operation (half-duplex or full-duplex), if the transmission via a relay node does not truly increase the capacity of the transmission, the relay operation is counter-productive.